Longitudinal vegetation turnover in an eastern Rift Valley riparian corridor

East African riparian corridors are biodiversity hot spots that occur in catchments degrading under deforestation and overgrazing. Quadrats and belt transects were employed to investigate plant succession along the River Gilgil, in the Kenyan Rift Valley. The study found that most species (total 365) were broadly distributed across the tropics, but Rift Valley endemics were 12.5%, and 4%–18% were exotic. Plant composition varied with altitude, lithology, soils and rainfall. Agglomerative (Jaccard) hierarchical clustering and Non-metric Dimensional Scaling identified two and three vegetation groups, for quadrats and transects, respectively, correlated with lithology and soils. Indicator species analysis highlighted that the upstream portion of the catchment was characterised by forest taxa with few exotics. The downstream included woodland species adapted to mesic/xeric conditions and to overgrazing. Longitudinal change in species composition (β diversity) between sites was assessed by taking into consideration the contribution of ‘spatial species turnover’ and ‘nestedness’, resulting from species replacement and species loss, respectively. The β diversity of the riparian corridor vegetation was 0.40 and spatial turnover accounted for 80%–85% of this, while the rest was attributed to the nestedness component. Riverine and native trees increased downstream but with low regeneration. Afromontane vegetation extended its distribution towards lower altitudes.